The planetary system Gliese 581 contains one of two planets named in the catalogue as potential homes for life. Artist's impression: AFP/Getty Images

A cosmic directory that lists the planets and moons most likely to harbour alien life was launched by astronomers on Monday.

Scientists created the online catalogue to make sense of the ever-rising number of distant worlds that researchers have spotted with modern telescopes.

They believe the database will help astronomers, and others with an interest, to compare faraway worlds and keep tabs on the most habitable ones as researchers discover them.

More than 700 "exoplanets" have been spotted and verified outside our own solar system in recent decades, while thousands more await confirmation by missions such as Nasa's Kepler space telescope.

The Habitable Exoplanets Catalogue in essence ranks the habitability of planets and moons according to three criteria: their surface temperature, similarity to Earth, and capacity to sustain organisms at the bottom of the food chain.

Only a small proportion of alien worlds appear ripe for life as we know it. The catalogue suggests that among hundreds of candidates, 15 or more planets and 30 moons are potentially habitable.

The catalogue gives high scores for habitability to two confirmed planets. The first, Gliese 581d, is among several that circle one of Earth's nearest stars, a cool red dwarf around 20 light years away in the constellation Libra. The planet is about six times as massive as Earth.

The second planet, HD85512b, orbits a star 36 light years away in the constellation Vela. It is more than three times as heavy as our own planet. Most of the planets astronomers have found are gas giants like Jupiter that are in close orbits around their stars.

Astronomers rank the planets by scoring them on three different scales. The first is called the habitable zone distance, which reflects the planet's position in the Goldilocks region of space around a star, where the conditions are neither too hot nor too cold for liquid water to form. Many astrobiologists consider liquid water essential for life to flourish.

The second scale is called the Earth similarity index, which ranks planets according to how closely their mass, radius, temperature and probability of having an atmosphere matches our own planet.

The third scale ranks planets according to their "global primary habitability", which reflects whether the estimated surface temperatures are suitable for life like plants and phytoplankton to grow. Earth scores quite low on this scale, because some basic organisms would fare better at warmer temperatures.

The database holds information on where the planets are, their probable mass, and the type and age of the star they orbit.

Future space telescopes, such as Nasa's proposed Terrestrial Planet Finder, have been designed to confirm whether alien worlds are suitable for life.

Details of the catalogue were due to be described at the Kepler science conference at Nasa's Ames facility in California on Monday.

"I hope this database will help increase interest in building a big space-based telescope to observe exoplanets directly and look for possible signatures of life," said Jim Kasting, who studies planetary habitability at Penn State University.